HEXYTHIAZOX RESISTANCE IN A FIELD POPULATION OF EUROPEAN RED MITE (ACARI, TETRANYCHIDAE) ON APPLES

Laboratory and field tests conducted from 1985 to 1989 demonstrated that a population of European red mite, Panonychus ulmi (Koch), in a apple orchard in Pennsylvania had become highly resistant to hexythiazox, a relatively new acaricide that is not registered for use on apples in the United States. A total of 20 treatments of hexythiazox had been applied to four different randomly selected trees throughout this 0.6-ha orchard during June and July from 1983 to 1986. Each year, 4-20 randomly selected trees were treated with two to five sprays of hexythiazox. In laboratory bioassays conducted in 1988 and 1989, mortality of overwintering eggs, summer eggs, and larvae avaraged < 40% at concentrations of hexythiazox ranging from 32 to 10,000 ppm (AI). In contrast, > 98% of these life stages of mites from a susceptible population of mites collected from a research orchard in New York were killed at hexythiazox concentrations of > 100 ppm (AI) in laboratory tests. The mites from both the New York and Pennsylvania populations were susceptible in laboratory bioassays to clofentezine (another new, chemically unrelated miticide which is also primarily active against mite eggs and larvae) despite the fact that 12 sprays of clofentezine were applied to the Pennsylvania population from 1981 to 1986.